Multi-stage piston compressor

ABSTRACT

A multi-stage piston compressor is provided with at least two piston-cylinder units, which respectively comprise a separate linear drive for moving the piston. A control device is also provided, which is designed in a manner such that the linear drives may be controlled individually with regard to speed and stroke.

BACKGROUND OF THE INVENTION

The invention relates to a multi-stage piston compressor having at least two piston-cylinder units.

Multi-stage piston compressors are known, in order to increase the pressure of a gas in several stages. With such multi-stage compressors, the exit pressure is increased in a first stage to a first intermediate pressure by a first piston-cylinder unit. This first intermediate pressure is then increased to a yet higher pressure in the second stage by a second piston-cylinder unit. This continues in accordance with the number of applied stages. With the known piston compressors, the pistons of all piston-cylinder units are driven via a common crank drive, which fixedly sets the stroke of the piston. Moreover, the diameter of the individual cylinders is selected differently, depending on the pressure ratio between the individual stages, i.e. the inner diameter of the cylinders reduces from stage to stage, since the volume of the gas decreases with each pressure increase. The known multi-stage piston compressors, on account of their mechanical design, are therefore set to certain stage pressure ratios, as well as a certain total volume flow and a certain total pressure. Variations are only possible in a restricted manner, by different valve control. For this reason, it is problematic to apply such piston compressors where different compressions or pressures and different volume flows are to be realized with one and the same machine.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the invention to improve a multi-stage piston compressor, to the extent that the pressure and volume flow may be varied in a simple manner with one and the same machine.

The multi-stage piston compressor according to the invention comprises at least two piston-cylinder units, i.e. it is designed at least in a two-stage manner. Correspondingly more piston-cylinder units are provided, in the case that more stages are envisaged. Thereby, at least one piston-cylinder unit is provided for each stage. According to the invention, each piston-cylinder unit is provided in each case with a separate linear drive for moving the piston. This means that the piston of the first piston-cylinder unit, i.e. of the first stage, is moved via a separate linear drive, and the piston of the second piston-cylinder unit, i.e. of the second stage, is driven by an individual separate linear drive. Thus one provides a separate linear drive for moving the piston for each stage, i.e. for each piston-cylinder unit which forms a stage. This means that here, one makes do without a common drive crank shaft and thus a mechanical coupling of the movement of the pistons of the individual piston-cylinder units. A synchronization or coupling of the movement of the individual pistons of the piston-cylinder unit, according to the invention, is effected in a purely control-technological manner by controlling the linear drives.

For this, according to the invention, a control device is provided, which is designed in a manner such that the linear drives may be controlled by the control device in an individual manner with regard to the speed and stroke. This means that the control device is designed such that it may activate the linear drives of the individual piston-cylinder units independently of one another, i.e. the stroke of the piston and also the movement speed of the piston may be separately controlled or set for each linear drive. Moreover, the starting point in time and the end point in time of the piston movement may be separately set for each piston-cylinder unit, and be changed as the case may be, by the control device suitably activating the respective associated linear drive of the piston-cylinder unit, i.e., by the control device beginning and stopping the piston movement.

One may set the multi-stage piston compressor to different delivery powers and pressures in a flexible manner due to the fact that one forgoes the mechanical movement coupling of the individual pistons. Thus, for example, the speed of the pistons may be varied, if a reduced volume flow is desired. Thereby, the stroke and speed of all pistons does not need to be increased or reduced to the same extent, but rather it is possible due to the inventive independent control of the drives, to vary the stroke and the speed of the individual pistons of the different stages independently of one another. In particular, it is possible to change the displacement volume by changing the stroke, for example of the second stage, in dependence on the pressure increase, which takes place in the first stage.

Preferably, the control device is thus designed in a manner such that the stage pressure ratio of the piston-cylinder unit may be changed by the individual control of the linear drives of the at least two piston-cylinder units. This is not possible with conventional piston compressors, which have a common crank shaft driving all pistons, since the pressure ratio and the volume ratio between the individual stages are matched to one another in a fixed manner. According to the invention, it is possible to change the stage pressure ratios of the individual stages by the mechanically independent drives of the nearest piston-cylinder units and the associated individual control. If the stage pressure ratio of a preceding stage, for example of the first stage, is increased, then accordingly, the displacement volume may be adapted by reducing the stroke in the subsequent stage, i.e., for example of the second stage. The stage pressure ratio thereby may likewise be changed by changing the stroke. Thereby, it is simultaneously possible to adapt the rates of stroke such that the individual stages, i.e. the individual piston-cylinder units, operate at the same frequency, even with a different stroke. Moreover, the dead space in the cylinder may be reduced, i.e. the volumetric losses may be reduced, by the individual control of the drives, since the upper and lower dead point may be moved to in a precise manner by the linear drive and the more precise control.

According to a preferred embodiment, the at least two piston-cylinder units are designed in a manner such that their cylinders have differently large inner cross sections, i.e. in particular different diameters. The associated pistons thereby are adapted with regard to their cross section or diameter to the inner cross section of the cylinders. By this design, in the second stage of the compressor, which must have a lower displacement volume than the first stage due to the higher pressure, one succeeds in this reduced displacement volume not only being achieved by changing the stroke by the linear drive, but also by a constructional adaptation of the piston-cylinder unit of the second and following stages. This means that the inner cross section preferably decreases from stage to stage. The volume ratio between the individual cylinders is mechanically predefined in this manner, but it does not thereby also automatically fix the pressure ratios or stage pressure ratios at the stages, on account of the separate linear drives. Rather, these may still yet be changed by changing the stroke of the respective linear drive by suitable programming or setting of the control device. Thus, expressed simply, a coarse volume adaption is achieved by the mechanical size graduation of the cylinders, while the individual fine adaptation is achieved by the control device by the individual control of the stroke of the linear drives.

Further preferably, the linear drives in each case comprise a rotating drive motor, in particular a servomotor, and a spindle drive, which converts the rotational movement of the drive motor into a linear movement for the piston. Sufficiently large forces may be applied onto the piston by such a drive. Furthermore, this may be controlled or regulated (controlled with a closed loop) in a precise manner with regard to its stroke and rate of travel. For this, positioning sensors may be provided on the piston, the spindle drive and/or the drive motor, in order to exactly detect the current position of the piston, and to precisely regulate the movement of the piston by control or regulation of the drive motor. The control device is accordingly designed to process the signals detected by the sensors, and to activate the drive motor while taking these signals into account. The spindle drive may be designed in a known manner, for example with a ball and screw. Preferably, the spindle drive is permanently lubricated, in particular lubricated for life, so than no continuous lubricant supply is necessary on operation. However, it is to be understood that other differently designed linear drives may also be applied, which are suitable for linearly moving a piston, and being activated individually by the control device. These in particular may also be other electrical linear drives. Thus, as the case may be, suitable gear means may be provided, in order to convert a rotating movement into a linear movement. However, the linear movement, according to the invention, is variable in stroke and speed by the control device.

Further preferably, the piston-cylinder units are designed in a dry-running manner. Thus, one may forego lubrication on operation, which significantly simplifies the complete construction of the compressor, and permits an application where a lubricant contamination of the gas to be compressed must be avoided.

According to a further preferred embodiment, an intermediate cooler may be arranged between two piston-cylinder units. This cools the compressed gas exiting from a first piston-cylinder unit, before it enters into the subsequent second piston-cylinder unit of the second stage of the compressor. Accordingly, such an intermediate cooler may also be arranged between a second and third, third and fourth stage etc., depending on how many stages the compressor has. Moreover, such a cooler may also be arranged at the exit side of the last stage.

Further preferably, the piston-cylinder units in each case may be designed in a double-stroke manner, so that a delivery or compression is effected with the forward stroke as well as backward stroke.

According to a particular embodiment of the invention, a switch-over valve is arranged on the exit side of at least a first piston-cylinder unit, by which valve the exit-side flow path may be switched between a subsequent second piston-cylinder unit and an exit conduit, preferably an exit conduit for several piston-cylinder units. This switch-over valve may also be actuated via the control device or, however, may be manually actuatable. Such a switch-over valve permits the piston compressor according to the invention to be converted from a multi-stage operation into a multi-flow operation, in which the individual piston-cylinder units of the several stages are not arranged in series, but are operated connected in parallel. Such an application may then be preferred if a larger volume flow is desired, with a lower pressure ratio between the entry pressure and the exit pressure of the compressor. It may also be possible, depending on the number of stages or piston-cylinder units in the piston compressor, not to connect all piston-cylinder units in parallel, but only individual ones. Moreover, by the use of such a switch-over valve, it is also possible to completely switch off individual piston-cylinder units. Thus, for example, with a three-stage piston compressor, the switch-over valve, which is arranged on the exit side of the second stage, i.e. of the second piston-cylinder unit, may be switched over such that the flow path no longer leads to the third piston-cylinder unit, but the flow is deflected directly into the exit conduit of the piston compressor. Then, for the thus separated or disconnected piston-cylinder unit, the linear drive is not set into operation at all by the control device. The efficiency of the compressor in such an operating mode may be increased in this manner, since the power loss may be reduced.

The control device is preferably designed as a memory-programmable control, at which certain exit parameters, in particular desired pressure and delivery volume, may be set. Moreover, the control device may be designed such that it processes the signals of different sensors and controls the linear drives of the individual stages or piston-cylinder units, while taking into account these parameters detected by the sensors. These may be, for example, pressure sensors or temperature sensors, which are arranged at the entry side and exit side of the compressor and/or between the individual stages of the compressor, in order to monitor the operating condition. The pressure signals and/or temperature signals may then, for example, be taken into account by the control device, in order to control the stroke and speed of the linear drives, such that desired defined pressure values or pressure ratios may be achieved by the compressor. Relatively or additionally, a monitoring of the volume flow in a corresponding manner would also be possible.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

FIG. 1 is a schematic, perspective view of a three-stage piston compressor according to one embodiment of the present invention;

FIG. 2 is a schematic, perspective view of a three-stage piston compressor according to a second embodiment of the invention;

FIG. 3 a schematic connection diagram of the compressor according to FIG. 2; and

FIG. 4 a schematic, perspective, sectional view of a cylinder usable in piston compressors of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The piston compressor shown in FIG. 1 is designed in a three-stage manner and accordingly comprises three piston-cylinder units 2, 4 and 6. The piston-cylinder units 2, 4, 6, as is shown in FIG. 4, consist in each case of a cylinder 8 which is cylindrical in cross section and of a piston 10 which is linearly movable therein. The piston 10 is connected to a piston rod 12. The cylinder 8 in a known manner comprises inlet valves and outlet valves, which may be designed as spring-biased check valves, which are suitably connected to the entry conduits and exit conduits.

According to the invention, each piston-cylinder unit 2, 4, 6 has its own linear drive 14, 16, 18. The piston-cylinder unit 2 comprises a linear drive 14 which is connected to the piston rod 12 of the piston cylinder unit 2, in order to move its piston 10 linearly in the inside of the cylinder 8. Correspondingly, the piston-cylinder unit 4 is connected to its own linear drive 16, which moves the piston 10 of the second piston-cylinder unit 4. The third piston-cylinder unit 6 has its own linear drive 18, which moves the piston 10 of the piston-cylinder unit 6 in its cylinder 8.

The linear drives 14, 16, 18 are designed as spindle drives, which are driven respectively by a servomotor 20, 22, 24. The servomotor 20 is assigned to the linear drive 14, the servomotor 22 to the linear drive 16, and the servomotor 24 to the linear drive 18. In this regard, each piston-cylinder unit 2, 4, 6 has its own independent drive for the respective piston. The pistons 10 may be moved in a very precise manner via the linear drives 14, 16, 18.

The servomotors 20, 22, and 24 of the linear drives 14, 16 and 18 are connected via electrical leads 26 to the control device 28, which activates or regulates the linear drives 14, 16, 18 or their servomotors 20, 22, 24.

An electronic coupling of the drives of the piston-cylinder units 2, 4, 6 may be achieved by the control device 28. Compared to a mechanical coupling, as is achieved with conventional piston compressors via the common crank shaft, this electronic coupling has the advantage that the coupling is variable and may be changed in the control device 28 according to different control programs or regulation programs. Thus, the stroke and rate of travel of each individual linear drive 14, 16, 18 may be set by the control device 28. This means that the pistons of each piston-cylinder unit 2, 4, 6 may be moved or controlled in their movement, independently of the respective other pistons. A significantly more flexible application of the compressor, with larger setting ranges with regard to pressure differences and volume flows, results on account of this.

The first piston-cylinder unit 2 comprises a gas inlet 30. The exit opening 32 of the first piston-cylinder unit 2 is connected via a conduit 33 to the entry opening 34 of the second piston-cylinder unit 4. The exit opening 36 of the second piston-cylinder unit 4 is connected via a conduit 37 to the entry opening 38 of the third piston-cylinder unit 6. The exit opening 40 of the third piston-cylinder unit is connected to a pressure conduit 42, which forms the exit conduit of the compressor. In this manner, the gas entering into gas inlet 30 is compressed in three stages in the piston-cylinder units 2, 4 and 6. Thereby, the pressure is increased in each stage. Since the volume decreases also in a corresponding ratio due to the pressure increase, the three piston-cylinder units 2, 4, 6 are designed with different cross-sectional sizes. The second piston-cylinder unit 4, which forms the second stage of the compressor, has a smaller cross section, i.e., a smaller inner diameter of the cylinder 8 and a smaller diameter of the associated piston 10, than with the first piston-cylinder unit 2. Correspondingly, the cross section of the third piston-cylinder unit 6 is once again smaller than that of the second piston-cylinder unit 4. These size changes preferably correspond to the ratio of the pressure increase from stage to stage, i.e., from piston-cylinder unit 2 to piston-cylinder unit 4, or from piston-cylinder unit 4 to piston-cylinder unit 6. Since the stage pressure ratio is variable with the system according to the invention, the size graduation in cross section of the piston-cylinder units 2, 4, 6 is preferably selected such that it corresponds to an average settable stage pressure ratio. Further volume variations may then be effected by the control of the stroke of the respective piston 10 via the individual linear drive 14, 16, 18. In contrast to systems driven by crank shafts, the stroke of the piston 10 according to the invention is specifically not predefined, but is variable via the associated linear drive 14, 16, 18.

The control device 28 is preferably designed as a memory-programmable control device and comprises a display and input device 44, which, for example, consists of a suitable keyboard and a display or of a touch-sensitive display. Alternatively or additionally, one may provide interfaces to computer systems. Alternatively, the control device 28 may also be integrated into a computer system, and the conduits 26 connected via suitable interfaces. Furthermore, the control device 28 may yet process different sensor signals as are explained by FIG. 3. Moreover, it may obtain position signals from the linear drives 14, 16, 18 and/or their servomotors 20, 22, 24, in order to be able to carry out an exact position regulation of the piston 10 in the cylinder 12 for each piston-cylinder unit 2, 4, 6.

It is possible to move the piston 10 in a very precise manner in the cylinder 12 by the linear drives 2, 4, 6. The dead volume or the waste space in the cylinder may be reduced thereby, and the piston 10 may be moved further toward the axial end wall of the cylinder 12 than with conventional systems, ideally almost completely up to the end wall. The volumetric efficiency of the system may be increased thereby. Moreover, the rate of travel of the piston 10 may be precisely set and controlled or regulated by the control device 28. Thereby, the rate of travel is preferably selected to be slower, preferably slower than 2 m/s, further preferably below 1 m/s. This leads to lower vibrations and to a low wear of the system.

Certain nominal values may be set on the compressor via the control device 28, in particular the desired exit pressure at the exit opening 40 of the third piston-cylinder unit 6. A desired volume flow may also be preset. The control device 28, depending on these variables, individually activates the three linear drives 14, 16, 18 via a control or regulation of the servomotors 20, 22, 24, so that the pistons 10 of the piston-cylinder units 2, 4, 6 respectively execute a desired stroke with a desired rate of travel. Thereby, the stage pressure ratio for each piston-cylinder unit 2, 4, 6, i.e., the pressure difference between the gas inlet 30 and the exit opening 32 of the first piston-cylinder unit 2, the pressure difference between the entry opening 34 and the exit opening 36 of the second piston-cylinder unit 4, and the pressure difference between the entry opening 38 and the exit opening 40 of the third piston-cylinder unit 6, may be individually set by the control device by a change of the stroke of the associated piston 10 by a suitable regulation of the linear drive 14, 16 and 18. The rate of travel is likewise correspondingly adapted by the control device 28, in order to ensure that all three piston-cylinder units 2, 4 and 6 operate with the same frequency, i.e., they require the same stroke time between the upper and lower dead points, even with a different length stroke. Thus, different stage pressure ratios and, in particular, a different total compression with different volume flows may be realized with the piston compressor according to the invention, without having to carry out constructional changes on the compressor. This is due to the variation of the individual stroke and the individual rate of travel of each of the individual piston-cylinder units 2, 4, 6 via the control unit 28.

With the embodiment shown in FIG. 1, switch-over valves 46 and 48 are arranged in the conduits 33 and 37 between the first piston-cylinder unit 2 and the second piston-cylinder unit 4, as well as between the second piston-cylinder unit 4 and the third piston-cylinder unit 6. This permits individual stages of the compressor to be completely disconnected. Thus, the switch-over valve 46 may switch the flow path at the exit side of the first piston-cylinder unit 2 between the entry opening 34 and the second piston-cylinder unit 4 and an exit conduit 50. The exit conduit 50 in the shown example is connected to the pressure conduit 42, but it is also conceivable for the pressure conduit 42 and the exit conduit 50 to be designed as a common exit conduit. If the switch-over valve 46 is switched such that the flow path runs to the entry opening 34 of the second piston-cylinder unit 4, the gas compressed in the first stage is thus led to the second piston-cylinder unit 4 as a second stage, in order to be further compressed there. If the switch-over valve 46 is switched to the exit conduit 50, the gas exiting from the exit opening 32 is no longer led to the second piston-cylinder unit 4, but directly into the pressure conduit 42. The compression with this setting is then effected only by the first piston-cylinder unit 2, and the second piston-cylinder unit 4 and the third piston-cylinder unit 6 are thus disconnected. In that case, the associated drives in the form of servomotors 22 and 24 may be switched off on account of the individual activation, so that the energy consumption may be reduced. The second switch-over valve 48 functions in the same manner. If this is switched such that the flow on the exit side of the exit opening 36 is led into the exit conduit 50, then gas is no longer led to the entry opening 38 of the third piston-cylinder unit 6, and this is set out of function. The compressor in this condition operates as a two-stage compressor with the piston-cylinder units 2 and 4.

The switch-over valves 46 and 48 may, however, permit yet a further operational mode, if specifically both switch-over valves 46 and 48 are switched such that the gas flow which exits the exit openings 32 and 36, is led directly into the exit conduit 50. If the switch-over valves 46 and 48 thereby simultaneously open an additional flow entry for the entry openings 34 and 38, the shown piston compressor may thus alternatively be applied as a three-flow compressor with which the three piston-cylinder units 2, 4 6 operate in parallel. A function as a two-flow compressor is also conceivable, if in this operating mode, one of the piston-cylinder units 2, 4, 6 is set out of operation by switching off the servomotor 20, 22 and 24 respectively. Instead of opening the additional gas entry also by the switch-over valves 46 and 48, one may provide separate valves for this.

FIG. 2 now shows an arrangement, which corresponds essentially to the arrangement according to FIG. 1. There, the three piston-cylinder units 2, 4 and 6 are, however, designed in a double-stroke manner in each case, i.e., the pistons 10 act with the forward stroke as well as with the return stroke. Moreover, in each case, intermediate coolers 52, 54 and 56 are arranged between the piston-cylinder units 2, 4 and 6, or at the exit side of the piston-cylinder unit 6. The intermediate cooler 52 is arranged in the conduit 33 between the piston-cylinder units 2 and 4, the intermediate cooler 44 in the conduit 37 between the piston-cylinder units 4 and 6, and the third intermediate cooler 56 on the exit side of the exit opening 40 of the third piston-cylinder unit 6. The intermediate coolers may be designed in a manner known per se as air coolers or water coolers. Due to the double-stroke construction, the first piston-cylinder unit comprises two entry openings 58 connected to the gas inlet 30, and correspondingly two exit openings 32 which are connected to the conduit 33 and the intermediate cooler 52 arranged in this. The conduit 33 then leads to two entry openings 34 of the second piston-cylinder unit 4. The second piston-cylinder unit 4 correspondingly comprises two exit openings 36 which are connected to the conduit 37 and the intermediate cooler 54 situated in this. The conduit 37 at the exit side of the intermediate cooler 54, leads to the two entry openings 38 of the third piston-cylinder unit 6. The third piston-cylinder unit 6 correspondingly comprises two exit openings 40, which lead to the intermediate cooler 56, which is connected at the exit side to the pressure conduit 42. The manner of functioning of the piston compressor according to FIG. 2 otherwise corresponds to the manner of functioning of the piston compressor according to FIG. 1, which is described above, only that it operates in a double-stroke manner. It is to be understood that the switch-over valves 46 and 48 with the connection to the exit conduit 50 may also be correspondingly used in the embodiment according to FIG. 2 and arranged in the conduits 33 and 37. These switch-over valves would then be usefully arranged on the exit side of the intermediate coolers 50 and 54.

FIG. 3 once again schematically shows the flow paths with a piston compressor according to FIG. 2. For simplification, the size differences between the piston-cylinder units 2, 4 and 6 are not represented in FIG. 3. The intermediate coolers are also not represented in FIG. 3, wherein it is to be understood that a double-stroke, three-stage compressor, as is shown in FIG. 2, may also be designed without an intermediate cooler. Alternatively, it would also be possible to provide suitable intermediate coolers with the compressor according to FIG. 1.

It is additionally shown in FIG. 3, that a multitude of temperature sensors T and pressure sensors P are provided in the system. These temperature and pressure sensors are provided on the entry side and the exit side of the piston-cylinder units 2, 4 and 6, in order to be able to detect the temperature and pressure at the gas inlet 30, in the conduit 33, the conduit 37, as well as the pressure conduit 42. Moreover, temperature sensors are yet provided at each exit of the piston-cylinder units 2, 4 and 6, so that the temperature of the gas compressed on the forward stroke of the piston 10 may be detected independently of the temperature of the gas compressed with the return stroke of the piston 10. The exit signals of the temperature sensors T and the pressure sensors P are likewise led to the control device 28 via suitable signal leads or other suitable signal transmission paths, and the control device takes these into account as actual values when regulating the linear drives 14, 16, 18 or their servomotors 20, 22, 24. In the case that intermediate coolers 52, 54, 56 are provided, it is also conceivable for the cooling power of the intermediate coolers to be able to be changed by the control device 28, for example by adapting the fan rotational speed of the coolers. Thus the cooling power may be adapted to the detected actual values of the temperature at the exit side of the individual stages 2, 4, 6.

Apart from the already mentioned advantages, the piston-cylinder units 2, 4, 6 are preferably designed in a dry-running manner in the shown examples, so that one may make do without lubrication. This is particularly advantageous if a contamination by lubricant of the gas to be delivered is to be prevented at all costs. The linear drives 14, 16, 18 may preferably be lubricated for life, so that here one need not provide for a continuous lubrication on operation. The linear drives 14, 16, 18 may, for example, be designed as spindle drives, in particular as ball and screw spindles.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 

1. A multi-stage piston compressor comprising at least two piston-cylinder units (2, 4, 6), each of the units having a separate linear drive (14, 16, 18) for moving the piston, and a control device (28) designed such that the linear drives (14, 16, 18) are controllable individually in speed and stroke.
 2. The multi-stage piston compressor according to claim 1, wherein the control device (28) is designed such that a stage pressure ratio of the piston-cylinder units (2, 4, 6) may be changed by the individual control of the linear drives (14, 16, 18) of the at least two piston-cylinder units (2, 4, 6).
 3. The multi-stage piston compressor according to claim 1, wherein the at least two piston-cylinder units (2, 4, 6) are designed such that cylinders (12) of the respective piston-cylinder units have differently large inner cross sections.
 4. The multi-stage piston compressor according to claim 1, wherein the linear drives (14, 16, 18) have respective rotating drive motors (20, 22, 24) and respective spindle drives, which convert a rotational movement of the drive motor (20, 22, 24) into a linear movement for respective pistons (10).
 5. The multi-stage piston compressor according to claim 1, wherein the piston-cylinder units (2, 4, 6) are designed as dry-running.
 6. The multi-stage piston compressor according to claim 1, further comprising respective intermediate coolers (52, 54, 56) arranged between the at least two piston-cylinder units (2, 4, 6).
 7. The multi-stage piston compressor according to claim 1, wherein the piston-cylinder units (2, 4, 6) are designed respectively as double-stroke.
 8. The multi-stage piston compressor according to claim 1, further comprising a switch-over valve (46, 48) arranged on an exit side of at least one of the first piston-cylinder units (2, 4), wherein the switch-over valve located at an exit-side flow path may be switched between a subsequent second piston-cylinder unit (4, 6) and an exit conduit (50). 